High-fidelity video coding tends to adopt RGB coding, which directly reduces RGB inter-color redundancy without external color space conversion. RGB coding is desirable for high resolution (HD) and accurate definition of saturation and hue. Several coding tools that support 4:4:4 format in RGB color space have been developed to improve RGB-domain 4:4:4 video coding efficiency. E.g. “Residue Color Transform”, ISO/IEC JTC1/SC29/WG11 and ITU-T Q6/SG16, Document JVT-L025, July 2004 by Woo-Shik Kim, Dmitry Birinov, and Hyun Mun Kim uses a residual color transform for 4:4:4 RGB coding, which first does intra/inter prediction to red (R), green (G) and blue (B) planes following the same procedure as H.264/AVC, and then adopts YCoCg-R color transform for the residual color transform. This has the disadvantage that a color residue transform generates much color distortion.
Some methods have been proposed to avoid color distortion resulting from the color space transform. The same authors as above present in “Color Format Extension”, ISO/IEC JTC1/SC29/WG11 and ITU-T Q6/SG16, Doc. JVT-H018, May 2003 and in “Inter-plane Prediction for RGB Coding II,” ISO/IEC JTC1/SC29/WG11 and ITU-T Q6/SG16, Doc. JVT-J017, December 2003 an inter-plane prediction (IPP) coding method for RGB coding. In this proposal, intra/inter prediction is first done to the three color components, and then the reconstructed G plane residue is used as a predictor block for each R and B residue. Finally, the predicted R residue and B residue follow the same coding procedure of the existing JVT specification as the G residue does. IPP coding avoids the color distortion resulting from the color space transform as well as coding error propagation. However, there are some cases where the correlation between color components is decreased after inter/intra prediction. In such cases IPP works poorly.
In another proposal, “Block Adaptive Inter-Color Compensation Algorithm for RGB 4:4:4 Video Coding”, IEEE Trans. Circuits and systems for video technology, vol. 18, no. 10, pp. 1447-1451, October 2008 by Byung Cheol Song, Yun Gu Lee and Nak Hoon Kim presents a block-based inter-colour compensation algorithm for removing inter-colour redundancy on RGB colour planes for video coding. It defines the correlation between colour planes within each coding block as a linear model of a slope and an offset parameter, codes the base colour plane block using the H.264/AVC standard, and then performs a weighted prediction to estimate the other two colour blocks from the base colour plane block. It uses a single common H.264 intra prediction to predict pixel values for calculating the offsets in its defined linear model for all blocks in a B/R plane. However, this predictor accuracy is often poor, and thus degrades linear prediction accuracy.
Further, Yong-Hwan Kim, Byeongho Choi, and Joonki Paik propose in “High-Fidelity RGB Video Coding Using Adaptive Inter-Plane Weighted Prediction” IEEE Trans. Circuits and systems for video technology, vol. 19, no. 7, pp. 1051-1056, July 2009 to use both intra-plane and inter-plane correlation between G/B, G/R and B/R planes in a coding block. For these two approaches, all of the sub-blocks within each colour component block share a single slope value. This is based on the assumption that the texture of a colour component within a current block is homogeneous and strongly correlated. However, for most coding blocks the texture within a component block varies quite much.